Tidal Signatures

According to Nio and Yang (1991) there are 4 unique features that can only be the result from tidal processes; “i) mud couplets or paired mud drapes; ii) lateral bundle/toeset thickness variation; iii) diurnal bundle thickness variations; and vi) reactivation surfaces”.

Shanley et al. (1992) emphasised the need for several structures to be present within the strata in order to determine the influence of tidal current activity. Visser (1980) claimed that double mudstone drapes are diagnostic of a subtidal depositional environment and that bundle sequences also are diagnostic for the tidal environment, reflecting different phases of the moon.

3.2.1 Tidal Bundles and Paired Mudstone Drape

Paired mudstone drapes, or double mud drapes, occur where sandstone laminae are separated by thin, mm, draping of mud or fine organic material (“coal drape”). The double mud drape represents the two slack-water stages in a daily tidal cycle between the dominant- and subordinate current. Double mud drape is a characteristic feature of subtidal environments as this depositional setting experiences two slack-water stages, whereas intertidal areas

experience only one, thus receive only a single mud drape, according to (Thomas et al. 1987;

Visser, 1980).

Double mud drapes often occur in cross-stratified sandstone of migrating dunes, with alternating foreset and toeset thickness. The dominant current will deposit a sandy foreset of a few centimetres thickness. The following first slack-water stage leaves a mud drape which often will be eroded at the top of the lee side during the subordinate current stage as the lee side then becomes the stoss side, i.e. a reactivation surface. The subordinate current will deposit a new sandy foreset, thought to be thinner than the one deposited during the dominant current, and this sand layer may have ripples climbing up the foreset. The second slack-water mud drape has a higher preservation value than the first one, as it forms at the dominant current’s lee side and can often be traced to the crest of the dune or ripple structure (Fig. 3.3). The mud drape couplet is visible since the dominant current deposits thicker foresets than the subordinate current. Mud-layer couplets are separated by sand deposited during the event of the dominant current, i.e. a tidal bundle. A tidal bundle is bounded below by the mud drape after the subordinate current, i.e. the second slack-water stage, and above by the mud drape after the dominant current stage, i.e. the first slack-water stage (Visser, 1980).

Fig. 3.3: The formation of double mud drapes and bundles in a subtidal setting.

A: Dominant current stage where thickest sandy foreset is deposited. B: First slack-water stage depositing a mud drape. C:

subordinate current stage, where erosion of first mud drape and possibly dominant stage sandy foreset occur, with deposition of a thinner sandy unit. D: Second slack-water stage with mud drape. From (Visser, 1980).

Single mud drapes within a fluvial system can represent fluctuations on the energy level in the stream, e.g. during a single flood cycle or alternating wet and dry periods and are thus not unique to the intertidal environment.

3.2.2 Bundle Sequence

A bundle sequence (terminology after Visser, 1980) represents the different moon phases, thus has a tidal signature and can be distinguished by the lateral variation in the thickness of the bundles, where the thick bundles represent the high tidal ranges around spring tide, and the thin bundles represent the low tidal ranges around neap tide (Visser, 1980). Normally such a sequence will have 26-28 bundles, representing a semi-diurnal (14,7 days neap-spring cycle) tidal regime. Due to the orbit of the Moon and the tilt of the axis of the Earth a diurnal (once a day) component is added to the tidal spectrum some places and may be dominant where the semidiurnal tide is small. A diurnal cycle will have 14 tidal bundles.

3.2.3 Flaser Bedding

Flaser bedding occurs in heterolithic channel fill, where daily tidal cycles drape sandy or silty ripples with mud. Fluctuating hydraulic conditions result in erosion of the ripple crests, leaving only the lower part of the ripple and mud between ripple sets preserved (Boggs, 2001, Shanley et.al. 1992).

3.2.4 Reactivation Surfaces

Reactivation surfaces are minor erosion surfaces which are gently dipping and slightly convex upstream of cross-stratified strata. Reactivation surfaces are present in fluvial, marine and tidal environments and can be due to fluctuation in stream flow in fluvial systems or reversal of flow due to tides, i.e. erosion of the lee side where the lee side becomes the stoss side (Shanley et al. 1992). Nio and Yang (1991) claimed that the occurrence of dominant flow reactivation surfaces together with those of subordinate currents is unique for the tidal environment. The rate of erosion will increase with increased strength of the subordinate

current and reactivation surfaces of different form and geometry will be formed. 5 different reactivation surfaces are suggested by Nio and Yang (1991) depending on erosion,

preservation of bundles and mud drape, development of back-flow ripples, dip of the reactivation surface and flaser development.

3.2.5 Bidirectional Current

Palaeocurrent measurements or structures showing a bidirectional current, with one dominant- and one subordinate direction, is a good tidal indicator. Such structures can be herringbone structures or ripples climbing up-dip on sandy foresets, indicating a dominant direction of migrating dunes and a subordinate direction (Nio and Yang, 1991).

3.2.6 Inclined Heterolithic Stratification (IHS)

Due to the amount of mud found in tidally influenced systems, Inclined Heterolithic

Stratification (IHS) is commonly found in relation to these, e.g. tidally influenced point bars, tidal channel deposits, deltaic distributary channels with tidal influence or lower delta plain levee deposits.

Thomas et al. (1987) suggested a genetic unbias terminology of Inclined Stratification (IS) and Inclined Heterolithic Stratification (IHS) regarding particularly lateral accretionary strata like point bars, though also other inclined strata like Gilbert-type deltas. This terminology could be used for inclined deposits independently of its depositional process and

environment in which it was formed. IHS is proposed for: “i) deposits with initial or

depositional dip; ii) of lithologically heterolithic composition, often with alternating coarser- and finer-grained units; iii) and with a wide variety of thicknesses”. IHS has in several cases been used to describe tidal deposits, e.g Shanley et al. (1992), tidally influenced point bar deposits; and Rebata et al. (2006), deposits within a tidal channel.

Thomas et al. (1987) emphasised, however, that IHS is not meant to be applied to tidal bundles, although mention that fine members of IHS often has been referred to as mud

to flood events and ephemeral streams in the fluvial system. Thus, tidally influenced point bar deposits can be classified as IHS, while any double mud drape or bundles present within these need to be described by its own terminology.